On 11/29/2014 3:59 PM, Ali Çehreli wrote:
[...] typeid() is a runtime function. I think it will be costly every time toHash is called. The function pointer can be initialized once.// I've "deduced" the type from an error message. ;) static const ulong delegate(const(void*)) const pure nothrow @trusted func; // This will be executed once per thread: static this() { func = assumePure(&(typeid(Foo.init.importantStuff).getHash)); } override size_t toHash() @trusted pure const nothrow { return func(&importantStuff); } Now there is no TypeInfo lookup after thread initialization time.
Nice! This is getting very fancy, but also a bit unfortunate. It would be appropriate if the type of importantStuff were arbitrary, but it's not. It's int[]. In fact, the fact that I can get a compiler-generated hash function for any type (I presume) surprises me. Not entirely, since the compiler can just treat types as binary blobs, and define a hash function that operates as if it were byte[].
Although it is nice that we can get at stuff through reflection, this is not quite the same as the ostensibly static [and inlinable] call to int[].toHash() which logically occurs when int[] is used directly as a key.
Also, would it be possible to avoid spelling the delegate type by making func a function static and using an auto return on a named method?
Finally, I find it a little surprising that there's no standard module which implements a quality hash function builder (using something like MurmurHash or CityHash). This seems like it would be a fairly common requirement for a lot of users. Are there any non-standard libraries which do this?
Dave
